Method for inhibiting premature vulcanization of rubber



Patented May 4 1948 METHOD FOR INHIBITING PREMATURE VULCANIZATION OF RUBBER Arthur W. Campbell, Terre Haute, Ind., assignor to Commercial Solvents Corporation, Terre Haute, Ind., a corporation of Maryland N 0 Drawing. Application January 1, 1945,

Serial No. 571,019 1 3 Claims.

This invention relates to a method for inhibiting the vulcanization of rubber material containing accelerators of vulcanization, and more particularly to a method for inhibiting vulcanization in the lower temperature range within which the rubber material is in a plastic, fiowable state.

In the process of vulcanizing rubber materials, accelerators of vulcanization are commonly used in combination with the vulcanizing agent to speed up the time of vulcanization and to impart various desired specific properties to the finished rubber and the like.

For some purposes, however, it is desirable to temporarily delay vulcanization of the rubber mix for a limited period while the rubber stock is in a plastic fiowable state. Such purposes include the operations wherein the mix is to be formed by molding and the mix is heated to a plastic state, and must flow and completely fill the mold before vulcanization and setting take place. Examples of such operations are those utilizing injection molding and the operations used in the manufacture of sponge rubber.

In the manufacture of sponge rubber, a gas producing material such as sodium bicarbonate is incorporated with the stock, and the batch is heated to a temperature sumciently high to cause the stock to become plastic and extensible, and at the same time to decompose the sodium-bicarbonate or other gas former, liberating gas such as carbon dioxide which expands within the batch to produce the cellular structure known as sponge. Ths operation is known as blowing in the trade. When even moderately active accelerators are used, considerable vulcanization may occur at the relatively low temperatures employed for blowing, resulting in premature stiffening of the stock which in turn results in an incomplete blow and in failure of the stock to fill the mold. Similar ill effects may occur where the stock is to be used for injection molding wherein a plasticized stock is injected into a hot mold, and in other processes where the stock must flow to a certain extent before setting. In injection molding the stock must remain plastic at least until the stock fills the hot mold, and premature vulcanization and stiffening of the stock before the stock is completely shaped results in unsatisfactory objects which must be rejected.

I have now found that premature vulcanization at moderately elevated temperatures at Which rubber stocks are plastic and fiowable can be avoided and yet the advantages of the accelerated stocks can be obtained at slightly more eleand the like, and the stock is milled in the usual way to form a homogeneous mass.

The amount of nitro alcohol required will vary somewhat with the particular nitro alcohol, the accelerator used and with the temperature and time during which vulcanization is to be delayed. In general I have found that a quantity of from about 0.5 to 10% of nitro alcohol based on the weight of rubber material present is sufficient for the usual molding operations, and quantities between 1 and 5% 0n the above basis are gen erally preferred. g

The nitro alcohols suitable for use as vulcanization retarders are any of the aliphatic nitro alcohols, for example, 2-nitro-1-propanol, l-nitro-2-propanol, 2-nitro-2-methyl-l-propanol, 2.- nitro-I-butanol, 3-nitro-2-butanol, 1-nitro-2'-' butanol, 3-nitro-4-heptanol, 2-nitro-2-methyl- 1,3-propanediol, 2-nitro-2-ethyl-L3-propanedio1,

tris(hydroxymethyl) nitromethane and the like.

In order to best secure the advantages of my invention, it is desirable to carry out the forming or molding process in two steps, the first-at a moderately elevated temperature high enough to induce good plasticity and flow characteristics of the stock, and yet not high enough for the inhibiting effect of the nitro alcohol on vulcanization to be overcome. Ordinary rubber stocks begin to become plastic in a temperature range considerably below that sufiicient for complete vulcanization, the temperature of vulcanization depending to some extent on the accelerator used. However, although only slight vulcanization usually takes place in the lower temperature ranges of plasticity, still this tendency is suflicient to markedly impair the flowing, mold filling characteristics in the case of injection molding and to impair the expansion and sponge forming as well as mold filling characteristics in the case of sponge rubber. 1

The inhibiting, effect on vulcanization of the nitro alcohols is efiective only in this lower range of rubber plasticity, andis overcome or destroyed at somewhat higher temperatures. Thus by the two stage molding process, the stock is first plasticized, and allowed to expand or flow so as to fill the mold and in the case of sponge rubber to assume the desired cellular structure, at the lower temperatures within the range of temperatures at which the stock is flowable, and then after these characteristics have been attained, the stock is subjected toa higher temperature at which the vulcanization inhibiting efiect of the nitro alcohol isovercome, and at this temperature the stock vulcanizes and sets in the desired shape or form.

The use of nitro alcohols iseffective to'inhibit' premature vulcanization of accelerated rubber stocks in which the accelerator is of the acidic type, for example, accelerators of th'e'thia'zol type and of the thiuram type such as benzothiazyl-v disulfide, tetramethyl-thiuram disulfi'de} Z-mercaptothiazol, 2-mercaptobenzothiazol, tetraeth ylthiuram disulfide, nitromercaptobenzothiaiol 'dium bicarbonate would, of course, beeliminated and the proportion of stearic acidwould be reduced. p

In the-tests described below, thenitro alcohol was added'in the proportions indicated based on 1110 parts of rubber in the test formula given be r- Tssr Fonmom I Parts Ruhber. 100 Zinc oxide 5 Sulfur 4 -Accelerat'or- '(benz'othiazyldisulfide) 1.25 Carboriblack 40 'Stearicacid Oi1n. 10 Sodium-bicarbonate Antioxidanflphenylp-naptliylamine) 2 In compounding the above stock, the rubber was"plas ticizedb'y 2 passesfthrough a tight mill arter which 'the-compounding ingredients, nameli'z'inobxide, sulfur, carbon black, accelerator andantioxidant were added in the customary manner, The compounded sheet was then furth' r plasticized by passstliiough a tight mill, after which the oil, sodium bicarbonate, stearic a'ci'd and nitro alcohol-were milled in, in the custernary manner. Sheets A" x 10" x 12" were curedin'a mold x 10" x 12" giving the stock an' opportunity to expend-to three times its ori irlalvolume. The stock was heated in the mold refer) minutes amt-260 F. to plasticize the-stock, decompose thesodium bicarbonate and allow the stac toexpand and fill the mold There should be no vulcanization during this curing and expansion perio'd. 'It-willlbe observed that with no nitio alcohol present, considerableprematurevulcanizationtookplace during the preliminary heating period, preventing, "filling, of the mold. In every casein which nitro alcohol was used, the vulcanization during thepreliminary heating period Iwas in'hibited to a greater or lesser extent, many cases resulting in a virtually complete 4 filling of the mold, to produce perfectly molded specimens.

After the preliminary heating period, the steam pressure on the molding platens of the test pieces was raised to 45 lb./sq. inch which corresponds to 293 F. This temperature was maintained for 30 minutes, whereupon the mold was opened and the sheet was recovered. The results are listed in the table below.

TABLE Inhibition of prcoulcanieatz'on of sponge rubber by various nitro olchools added to test formula given 'dbo'oe Amount Nitro alcohol g g Blow Molding Rubber 2-Nitro-2-metl1yl-l-propanol (solid) 5 Good Perfect. Do 3.2 do-. Good. Do 1.6 do Perfect. Do. 0.8 do Good. 2-Nitro-2-methyl-l-propanol (25% solution) 5 do.. Excellent Z-Nitro-l-butanol 5 do Fair. 2-Nitro-3-butanol 5 -do Do. 3-Nitro-4-heptonol 5 do. Do. 2-Nitro-2-methyl-l;3-propanediol 5 ,do Do. 2-Nitro-2-ethyl-l,3-propanedioln 5 .d0l Do. Tris (hydroxymethyl) nitromethane 5 -,do-.. Perfect. None (Control) None do Very poor.

While the above describes the preferred embodiments of my'invention, it will be understood that departures may be made therefrom within the scope of the specification and claims.

What is claimed is:

1. A process for preparing sponge rubber which comprises adding a lower substituted nitro alkanolihaving the structural formula R -orn0n where Rand R have the same values and are selected .from the group consisting of CH3 and CHeOI-I, to a rubberstockcontaining an acidic acceleratorand a gas forming ingredient, heating the stock at a temperature high enough to pro duce plasticity of the rubber stock and decomposition of the gas forming ingredient, but low enough to be within the range within which the vulcanization inhibiting effect "of the said lower substituted nitro alkanol is operative, maintaining said stock at said temperature until the desiredccmplete expansion of the stock has taken place, and thereafter raising the temperature of the stock to a temperature at which the inhibiting efiect of the said lower substituted nitro alkanol is overcome, whereby the stock is vulcanized and set in its completely expanded sponge form.

2. A process-for preparing sponge rubber which comprises addingw 2-nitro-2-methyl-1-propanol to a rubber stock containing an acidic accelerator and a gas-forming ingredient, heating the stock at. a temperature high enough to produce plasticity of the rubber stock and decomposition of the gas-forming ingredient, but low enough to be within the. range within which the vulcanization inhibiting efiect of the 2-nitro-2-methyl-1-propanel is operative, maintaining said stock at said temperature untilthe desired complete expansion of the stock has taken place, and thereafter raising the temperature of the stock to a temperature at which the inhibiting efiect of the said z-nitro-z-mehyl-l-propanol is overcome, where:

comprises enough to be within the range within which the 10 vulcanzation inhibiting efiect of the tris(hydrodroxymethybnitromethane is operative, maintaining said stock at said temperature until the desired complete expansion of the stock has taken place, and thereafter raising the temperature of the stock to a temperature at which the inhibiting effect of the said tris(hydroxymethyl') nitromethane is overcome, whereby the stock is vulcanized and set in its completely expanded sponge" form.

ARTHUR W. CAMPBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,998,559 Sibley Apr. 23, 1935 2,231,950 Roberts et al Feb, 18, 1941 15 2,297,871 Campbell Oct. 6, 1942 2,304,335 Campbell Dec. 8, 1942 

